Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
In one approach, as shown by Glugla et al. in US 20120245827, in an engine system configured to receive fuel via direct injection, in response to an indication of pre-ignition, the engine is operated in a split injection mode. Specifically, the pre-ignition affected cylinder is enriched by providing a rich fuel injection over multiple direct injections instead of a single direct injection. Fueling of one or more other cylinders is then adjusted to maintain an exhaust air-fuel ratio at or around stoichiometry.
However the inventors herein have recognized issues with such an approach. While the charge cooling effect of the direct injection improves pre-ignition mitigation, it also generates more particulate matter emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. Similarly, the injected fuel may encounter less turbulence when flowing through the valves. Consequently, there may be pockets of rich combustion that may generate soot locally, degrading exhaust emissions. Since the pre-ignition mitigating direct injection is a rich fuel injection, the propensity of degraded emissions is higher. The inventors have further recognized that in engine systems configured with port and direct injection systems, the charge cooling properties of the port injection system can also be leveraged to address pre-ignition. In particular, the charge cooling properties of a port injection performed on an open intake valve can be used to provide at least some of the pre-ignition mitigating cylinder cooling without incurring significant particulate matter emissions.
Thus in one example, pre-ignition mitigation may be improved in an engine system configured for port and direct injection of fuel. The method may comprise: in response to an indication of pre-ignition, enriching a cylinder by increasing a ratio of port injected fuel relative to direct injected fuel for a number of enrichment cycles.
As an example, in response to an indication of pre-ignition, on an immediately subsequent engine cycle, the pre-ignition affected cylinder may be enriched by increasing a ratio of fuel delivered to the cylinder via port injection. For example, the pulse width of the port injector may be increased, if possible. In addition, the port injection may be timed to occur during an open intake valve event to increase the charge cooling effect of the port injected fuel. At the same time, direct injection of fuel may be also be increased. As an example, an engine may be operating with a portion of the fuel requirement delivered via port injection on a closed intake valve (e.g., during an exhaust stroke) and a remaining portion of fuel requirement delivered via direct injection during an intake stroke and/or a compression stroke. In response to the indication of pre-ignition, the amount of fuel port injected may be increased while the timing of port fuel injection is shifted to an open intake valve (e.g., during an intake stroke). In addition, the amount of fuel that is direct injected is also increased with the portion of fuel injected in the intake stroke increased and the portion of fuel injected in the compression stroke decreased. For example, fuel may be direct injected in the intake stroke only and no fuel may be direct injected in the compression stroke. If the pulse width of the port injector cannot be increased on the immediately subsequent engine cycle, increased direct injection of fuel may be used initially to address the pre-ignition. The relative increase in port injection may be higher than the relative increase in direct injection for a first number of enrichment cycles (e.g., the first enrichment cycle following the indication of pre-ignition). Thereafter, a ratio of fuel delivered as a port injection relative to a direct injection may be adjusted over a number of enrichment cycles to increase charge cooling. For example, fuel may be delivered with a higher ratio of direct injected fuel relative to port injected fuel for a number of engine cycles until a pre-ignition propensity falls. Thereafter, nominal engine fueling may be resumed.
In this way, by adjusting the ratio of an enriched fuel injection delivered to an engine via port injection relative to direct injection, the charge cooling properties of a port injection can be better leveraged for pre-ignition mitigation. By injecting at least some fuel via a port injector on an open intake valve event in response to an indication of pre-ignition, exhaust particulate matter emissions may be reduced. Specifically, by trading off a portion of the enrichment that was to be direct injected with fuel that is port injected, pre-ignition can be addressed without increasing exhaust particulate matter emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.